One area of nanotechnology is to develop chemical building blocks from which hierarchical macromolecules of predicted properties can be assembled. An approach to making chemical building blocks or nanostructures begins at the atomic and molecular level by designing and synthesizing starting materials with highly tailored properties. Precise control at the atomic level is the foundation for development of rationally tailored synthesis-structure-property relationships which can provide materials of unique structure and predictable properties. This approach to nanotechnology is inspired by nature where, for example, from twenty common amino acids found in natural proteins, more than 105 stable and unique proteins are made.
Nanotechnology has also been described by K. Eric Drexler in Engines of Creation as “the knowledge and means for designing, fabricating and employing molecular scale devices by the manipulation and placement of individual atoms and molecules with precision on the atomic scale.” A quest of nanotechnology is to prepare molecular architectures capable of performing on a nanometer scale functions normally observed for large-scale constructs. For example, rotaxanes and polyrotaxanes are molecules that are interlocked, but not chemically bound to one another, which act like nano-machines. In other examples, carbon nanotubes and similar constructs have been created which may function as molecular scaffold units, or as transport channels, storage units, or encapsulators for various atoms and molecules. The use of biological processes is also being studied as an approach to the assembly of non-biological nano-devices.
A hurdle in developing “building block” nanotechnology is creating the ability to program the final output of a chemical reaction between the reactants from which the building blocks are formed. In other words, it is desirable to control the geometry of the building block reactants in order to predetermine which product will be built from the reactants. The product will be the thermodynamically favored product in most cases, however, the programming of geometrical constraints into the reactants overrides the random statistics of bulk phase macroscopic interactions and effectively limits the reaction at the atomic level.
Entropically driven self assembly processes may be used to produce hierarchical products which are typically well-organized aggregates. Although such processes may be robust in tolerating a range of conditions, they are generally very limited in synthetic accessibility and produce a narrow range of products.
Most conventional methods of preparation of organic compounds involve stepwise attachment of species to form a product. Step-by-step synthesis can be an arduous route to prepare molecules, even some relatively simple molecules. Random statistics plays a role in every step and can make it impossible to achieve multistep products. For example, a collection of cyclic synthons may be coupled in stepwise synthesis to yield macrocyclic modules, however, the yield in many cases is relatively low, or may even be prohibitive.
One application that will benefit from nanotechnology is filtration using membranes. Conventional membranes used in a variety of separation processes can be made selectively permeable to various molecular species. The permeation properties of conventional membranes generally depend on the pathways of transport of species through the membrane structure. While the diffusion pathway in conventional selectively permeable materials can be made tortuous in order to control permeation, porosity is not well defined or controlled by conventional methods. The ability to fabricate regular or unique pore structures of membranes is a long-standing goal of separation technology.
Thus, what is needed is an approach to making chemical entities in the form of bridged macrocyclic module compositions from which to create nanostructures with desirable properties.
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